Control method for use with a steerable drilling system

ABSTRACT

A control method for use with a steerable drilling system comprises the steps of inputting parametric model data representative of drilling conditions and using the data to determine achievable drilling directions.

This application claims benefit of provisional application No.60/164,681 filed Nov. 10, 1999.

This invention relates to a method for use in controlling the operationof a steerable drilling system. The method is particularly suitable foruse with a rotary steerable system, but may be used in other types ofsteerable drilling system used in the formation of subterranean wells.

One type of rotary steerable system comprises a downhole assemblyincluding a drill bit. The drill bit is carried by a drill string whichis rotated typically by a well head located drive arrangement. A biasunit is included in the downhole assembly, the bias unit including aplurality of hinged pads moveable between extended and retractedpositions. The pads are moved hydraulically using drilling fluid underthe control of a valve arrangement. The valve arrangement is designed topermit control over the pads such that, when desired, the pads can bemoved to their extended positions in turn as the bias unit rotates. Byappropriate control over the pads, the bias unit can be operated toapply a sideways load on the drill bit which in turn will cause theformation of a curve in the well bore being drilled. The orientation ofthe curve will depend upon how the bias unit is controlled.

It has been found that a number of factors must be taken into accountwhen controlling the operation of a rotary steerable system. Forexample, the rate of change of direction of the bore hole being formedin response to the application of a given command signal to the biasunit depends upon several factors associated with the drilling system,for example rotary speed, weight on bit, rate of penetration and severalfactors associated with the formation being drilled, for example the dipand azimuth of bedding planes. As a consequence, it is common for wellbores drilled using steerable drilling systems to deviate from theirdesired paths. Such well bores may be of tortuous form containing manydog legs. Depending upon the orientation of the curves formed in thewell bore, water or gas may tend to collect in the curves. Suchaccumulation of water or gas may impair subsequent use of the well borein the extraction of oil.

It is an object of the invention to provide a control method for usewith a steerable drilling system, the method simplifying control of thedrilling system.

According to the present invention there is provided a method ofcontrolling the operation of a steerable drilling system comprising thesteps of:

inputting parametric model data representative of drilling conditions;

inputting data representative of a desired drilling direction; and

using the parametric model data and the data representative of thedesired drilling direction in controlling the operation of the steerabledrilling system.

The parametric model data is conveniently derived using data collected,in real time, during drilling. The parametric model data may includedata representative of one or more of the following parameters: weighton bit, rotational speed, rate of penetration, torque, pressure,inclination, dip and azimuth of bedding planes or other formationcharacteristics, hole curvature/gauge or other geometric conditions, bittype and condition, and errors in instrumentation readings.

The use of such a system is advantageous in that compensation can bemade for the operating conditions, thus the risk of supplying thedrilling system with instructions to drill a curve of too tight or toosmall a radius of curvature or of too great or small a length in a givendirection can be reduced, thus permitting the drilling of a well bore ofless tortuous form.

The parametric model data and data representative of the desireddrilling direction may be used directly in controlling the operation ofthe drilling system. Alternatively, an output signal may be producedwhich is used to control a display to provide an operator withinformation for use in controlling the operation of the drilling system.The display may be in a graphic form, for example in the form of a graphof build rate response against turn rate response upon which is plottedan envelope indicating the achievable responses for one or more sets ofoperating conditions.

With such a display, an operator will be able to see whether it ispossible to steer the drill bit of the drilling system in a givendirection under one or more sets of operating conditions. The operatormay then be able to modify one or more of the operating conditions overwhich he has some control to ensure that the operating conditions underwhich the drilling system is operating are such as to permit steering ofthe drill bit in the desired direction.

The invention will further be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a diagram illustrating a drilling installation, with which themethod of the invention may be used,

FIG. 2 is a sectional view illustrating part of the downhole assembly ofthe installation of FIG. 1,

FIG. 3 is a flowchart illustrating a method in accordance with anembodiment of the invention, and

FIG. 4 is a representation of an output achieved using the methoddescribed with reference to FIG. 3.

FIG. 1 shows diagrammatically a typical rotary drilling installation ofa kind in which the methods according to the present invention may beemployed.

In the following description the terms “clockwise” and anti-clockwise”refer to the direction of rotation as viewed looking downhole.

As is well known, the bottom hole assembly includes a drill bit 1, andis connected to the lower end of a drill string 2 which is rotatablydriven from the surface by a rotary table 3 on a drilling platform 4.The rotary table is driven by a drive motor, indicated diagrammaticallyat 5, and raising and lowering of the drill string, and application ofweight-on-bit, is under the control of draw works, indicateddiagrammatically at 6.

The bottom hole assembly includes a modulated bias unit 10 to which thedrill bit 1 is connected and a roll stabilised control unit 9 whichcontrols operation of the bias unit 10 in accordance with signalstransmitted to the control unit from the surface. The bias unit 10 maybe controlled to apply a lateral bias to the drill bit 1 in a desireddirection so as to control the direction of drilling.

Referring to FIG. 2, the bias unit 10 comprises an elongate main bodystructure provided at its upper end with a threaded pin 11 forconnecting the unit to a drill collar, incorporating the roll stabilisedcontrol unit 9, which is in turn connected to the lower end of the drillstring. The lower end 12 of the body structure is formed with a socketto receive the threaded pin of the drill bit.

There are provided around the periphery of the bias unit, towards itslower end, three equally spaced hydraulic actuators 13. Each hydraulicactuator 13 is supplied with drilling fluid under pressure through arespective passage 14 under the control of a rotatable disc valve 15located in a cavity 16 in the body structure of the bias unit. Drillingfluid delivered under pressure downwardly through the interior of thedrill string, in the normal manner, passes into a central passage 17 inthe upper part of the bias unit, through a filter, and through an inlet19 to be delivered at an appropriate pressure to the cavity 16.

The disc valve 15 is controlled by an axial shaft 21 which is connectedby a coupling 22 to the output shaft of the control unit, which can beroll stabilised.

The control unit, when roll stabilised (i.e. non-rotating in space)maintains the shaft 21 substantially stationary at a rotationalorientation which is selected according to the direction in which thedrill bit is to be steered. As the bias unit rotates around thestationary shaft 21 the disc valve 15 operates to deliver drilling fluidunder pressure to the three hydraulic actuators 13 in succession. Thehydraulic actuators are thus operated in succession as the bias unitrotates, each in the same rotational position so as to displace the biasunit laterally in a selected direction. The selected rotational positionof the shaft 21 in space thus determines the direction in which the biasunit is actually displaced and hence the direction in which the drillbit is steered.

If the shaft 21 is not held in a substantially stationary position, thenthe actuators 13 are operated in turn but are not all operated in thesame rotational position. As a result, rather than urging the bias unitlaterally in a given direction, the direction in which the bias unit isurged changes continuously with the result that there is no net biasapplied by the bias unit.

Drilling systems of the general type described hereinbefore aredescribed in greater detail in EP 0520733, EP 0677640, EP 0530045, EP0728908 and EP 0728909, the content of which is incorporated herein byreference.

As described hereinbefore, for a given biasing load applied by the biasunit, the rate of change of direction of the bore being formed isinfluenced by a number of factors. The factors influencing the verticalrate of change, the build rate, are not always the same as thoseinfluencing the rate of change in the horizontal direction, known as theturn rate.

FIG. 3 is a flowchart illustrating a method of controlling the operatingof the drilling system of FIGS. 1 and 2. As shown in FIG. 3, at thestart of drilling a control system used in controlling the positionoccupied by the shaft 21 is initialised with data representative of thelikely drilling conditions. The input data is representative of factorsassociated with the drilling system, the formation being drilled, thedirection of the well bore, and the shape of the well bore. The factorsassociated with the drilling system include the intended weight on bit,rate of penetration, rotational speed, torque, pressure and inclinationof the drill bit. The factors associated with the formation beingdrilled include the dip and azimuth of bedding planes. Datarepresentative of likely errors in sensor readings and representative ofthe type and condition of the drill bit may also be input. If nosuitable data is available to be input, then a default data set may beused.

Whilst drilling is taking place, data representative of the actualdrilling conditions is collected and transmitted to the control system.The readings are conveniently taken at intervals, for example at every30 metres of measured depth. The measured data is used to update thedata of the parametric model.

The updated data set of the parametric model is used to calculate arange of achievable drilling directions, and this information isdisplayed graphically to the operator of the drilling system, forexample in the form of a chart as shown in FIG. 4. As shown in FIG. 4,the chart takes the form of a graph of build rate against turn rate uponwhich is plotted an envelope 25 illustrating the achievable drillingdirection for the prevailing drilling conditions. Also plotted on thegraph is the current drilling direction 26. The chart may also indicatea desired drilling direction 27 if this information has been input bythe operator. Such a desired drilling direction 27 is indicated on FIG.4.

Using the information displayed, the operator can determine whether ornot it is possible to achieve the desired drilling direction 27 underthe prevailing drilling conditions. This is a relatively simple task as,if the desired drilling direction 27 falls within the envelope 25, thenit is achievable with the current drilling conditions, and drilling cancontinue with appropriate signals sent to the bias unit to urge thedrill bit to drill in the desired direction.

If the desired drilling direction 27 falls outside of the envelope 25 ofachievable directions (as shown in FIG. 4), then obviously if the wellbore is to be drilled in the desired direction, this can only beachieved if the drilling conditions change. Although the operator has nocontrol over a number of the drilling conditions, in particular thedrilling conditions governed by the formation, he does have control oversome of the drilling conditions associated with the operation of thedrill bit. For example, the operator could modify the rate ofpenetration, weight-on-bit, or rotational speed of the drill bit. Priorto modifying the drilling conditions, the operator may input trialvalues of certain of the operating parameters into the control system.The control system is arranged to display the envelope 28 of achievabledrilling directions for those operating conditions. If the trial valuesfor the operating conditions result in the production of an envelope ofachievable drilling directions including the desired drilling direction27, then the operator may choose to use those drilling parameter valuesin the control of the drilling system and then to direct the drill bitin the desired direction. Alternatively, the control system may be setup in such a manner as to output suitable values for the drillingparameters in response to the operator entering a desired drillingdirection.

A number of different algorithms may be used in the calculation of theenvelope of achievable drilling directions.

In one simple technique, the response of the system to a given input isused to calculate gain values K_(B) and K_(T), cross-coupling valuesC_(BT) and C_(TB) and bias values B_(bias) and T_(bias) (where B and Trepresent Build and Turn respectively).

The build and turn values are then calculated by, for each factorinfluencing the responsiveness of the system to a steering command,calculating a normalised deviation of the parameter value from the meanvalue of that parameter and multiplying the deviation by a coefficientrepresentative of the responsiveness of the system to that one of thefactors, and adding the results for each factor to one another and tothe relevant ones of the gain, cross-coupling and bias values. Thesecalculations can be expressed by the following equations:$\begin{matrix}{{Build} = \quad {{W_{build}*\left\lbrack \frac{{WOB} - {meanWOB}}{meanWOB} \right\rbrack} + {R_{build}*\left\lbrack \frac{{ROP} - {meanROP}}{meanROP} \right\rbrack} +}} \\{\quad {{P_{build}*\left\lbrack \frac{{Pressure} - {meanPressure}}{meanPressure} \right\rbrack} + {F_{build}\left\lbrack \frac{{Flow} - {meanFlow}}{meanFlow} \right\rbrack} +}} \\{\quad {{M_{build}*\left\lbrack \frac{{RPM} - {meanRPM}}{meanRPM} \right\rbrack} + {T_{build}*\left\lbrack \frac{{Torque} - {meanTorque}}{meanTorque} \right\rbrack} +}} \\{\quad {{I_{build}*\left\lbrack \frac{{\sin \quad {lnc}} - {{mean}\quad \sin \quad {lnc}}}{{mean}\quad \sin \quad {lnc}} \right\rbrack} + {K_{B}*\left\lbrack {{BuildDemand}\%} \right\rbrack} +}} \\{\quad {{C_{BT}*\left\lbrack {{TurnDemand}\%} \right\rbrack} + {build}_{bias}}}\end{matrix}$ $\begin{matrix}{{Turn} = \quad {{W_{turn}*\left\lbrack \frac{{WOB} - {meanWOB}}{meanWOB} \right\rbrack} + {R_{turn}*\left\lbrack \frac{{ROP} - {meanROP}}{meanROP} \right\rbrack} +}} \\{\quad {{P_{turn}*\left\lbrack \frac{{Pressure} - {meanPressure}}{meanPressure} \right\rbrack} + {F_{turn}\left\lbrack \frac{{Flow} - {meanFlow}}{meanFlow} \right\rbrack} +}} \\{\quad {{M_{turn}*\left\lbrack \frac{{RPM} - {meanRPM}}{meanRPM} \right\rbrack} + {T_{turn}*\left\lbrack \frac{{Torque} - {meanTorque}}{meanTorque} \right\rbrack} +}} \\{\quad {{I_{turn}*\left\lbrack \frac{{\sin \quad {lnc}} - {{mean}\quad \sin \quad {lnc}}}{{mean}\quad \sin \quad {lnc}} \right\rbrack} + {K_{T}*\left\lbrack {{TurnDemand}\%} \right\rbrack} +}} \\{\quad {{C_{TB}*\left\lbrack {{BuildDemand}\%} \right\rbrack} + {turn}_{bias}}}\end{matrix}$

As mentioned above, other mathematical techniques may be used in thederivation of the envelopes of achievable steering directions.

Rather than use the method to determine which steering directions areacheivable for a given set of drilling conditions, or to determine setsof drilling conditions which can be used to acheive steering in a chosendirection, the method may be used to determine acheivable rates ofpenetration for a given set of drilling conditions. Such use of themethod may have the advantage that the rate of penetration can beoptimised.

Although the description hereinbefore related to the use of a specifictype of steerable system, it will be appreciated that the invention isnot restricted to the use of the method with the described drillingsystem and that the invention could be used with a range of otherdrilling systems.

What is claimed is:
 1. A method of controlling the operation of asteerable drilling system comprising: inputting parametric model datarepresentative of drilling conditions; calculating build and turn gain,cross-coupling and bias values using the parametric model data; usingthe calculated build and turn gain, cross-coupling and bias values toderive build and turn responsiveness values; using the derived build andturn responsiveness values in controlling the operation of a steerabledrilling system; wherein an output signal is produced which is used tocontrol a display to provide an operator with information for use incontrolling the operation of the drilling system and wherein the displayis in graphic form of build rate response against turn rate responseupon which is plotted an envelope indicating the achievable responsesfor one or more sets of operating conditions.
 2. The method of claim 1further comprising: inputting data representative of a desired drillingdirection; and using the parametric model data and the datarepresentative of the desired drilling direction in controlling theoperation of the steerable drilling system.
 3. The method of claim 2wherein the parametric model data and the desired drilling directiondata are used directly in controlling the drilling system.
 4. The methodof claim 1, wherein data collected during drilling is used to update themodel.
 5. The method of claim 1, wherein the model uses datarepresentative of at least one of: weight on bit, rotational speed, rateof penetration, torque, pressure, inclination, dip and azimuth ofbedding planes or other formation characteristics, hole curvature/gaugeor other geometric conditions, bit type and condition, and errors ininstrumentation readings.
 6. A method of controlling the operation of asteerable drilling system comprising the steps of: inputting parametricmodel data representative of drilling conditions; calculating build andturn gain, cross-coupling and bias values using the parametric modeldata; using the calculated build and turn gain, cross-coupling and biasvalues to derive build and turn responsiveness values; using the derivedbuild and turn responsiveness values to calculate a range of achievabledrilling directions.
 7. The method of claim 6 wherein the drillingconditions are selected from the group consisting of weight on bit, rateof penetration, rotational speed, torque, pressure, inclination of thedrill bit, dip and azimuth of bedding planes or other formationcharacteristics, hole curvature/gauge or other geometric conditions, bittype and condition, and errors in instrumentation readings.
 8. Themethod of claim 6, further comprising outputting the calculated range ina graphic form.
 9. The method of claim 6, further comprising inputting adesired drilling direction, and using the derived build and turnresponsiveness values in determining whether drilling in the desireddrilling direction is achievable.
 10. A method of controlling theoperation of a steerable drilling system comprising: inputtingparametric model data representative of drilling conditions; inputtingdata representative of a desired drilling direction; calculating buildand turn gain, cross-coupling and bias values using the parametric modeldata; using the parametric model data and the data representative of thedesired drilling direction to produce an output signal; and using theoutput signal to control a display to provide an operator withinformation in a graphic form for use in controlling the operation ofthe drilling system, wherein the display is in the form of a graph ofbuild rate response against turn rate response upon which is plotted anenvelope indicating achievable responses for one or more sets ofoperating conditions.
 11. The method of claim 10 wherein the drillingconditions are selected from the group consisting of weight on bit, rateof penetration, rotational speed, torque, pressure, inclination of thedrill bit, dip and azimuth of bedding planes or other formationcharacteristics, hole curvature/gauge or other geometric conditions, bittype and condition, and errors in instrumentation readings.